SELECT s.id, count(z.mac), sum(z.c) FROM snmp_ports s
    LEFT OUTER JOIN
        (
        SELECT  s.id, f.mac, case when count(a.mac)>0 then 1 else 0 end as c from snmp_ports s
            LEFT OUTER jOIN fdb f on (f.port = s.id AND f.last_seen > now()-'30 days'::interval)
            left outer join arp a on (f.mac = a.mac)
            where s.equipment=3097
            group by s.id, f.mac) as z ON s.id=z.id
    
    where s.equipment = 3097
        group by s.id order by s.id